Ultrasonic examinations are performed within the nuclear industry and most other major industries to determine the condition of parts and components. The metal or alloy material of a part or component is inspected using ultrasound to detect any flaws which could prove detrimental to the safe operation of that part or component. The ultrasonic NDE method can be used to detect internal flaws in most engineering metals and alloys. Bonds produced by welding, brazing, soldering and adhesive bonding can also be ultrasonically inspected.
Ultrasonic inspection is used for quality control and materials inspection in the fabrication of structures, reactor pressure vessels, airframes, pipe systems, bridges, motor vehicles and jet engines. The present invention has application in all of these fields.
For successful application of ultrasonic examination techniques, the ultrasonic system, including transducers, must be suitable for the type of inspection being performed. If the proper transducer is not used, there is a high potential for gross error in the inspection results, or there could be no results at all. For instance, using a common ultrasonic transducer that has a hard flat-surfaced Lucite wedge for examining as-welded overlaid pipe welds results in gross errors in the ultrasonic inspection results. In many cases ultrasonic inspection data is not recorded at all. This is due to the presence of air gaps between the transducer head and the rough surface being inspected, which forms an opaque barrier.
Ultrasonic characterization of cracks in materials is at least a two-step process: 1) detection and location; and 2) sizing in absolute or relative terms. In accordance with the first step of this process, the transducer is excited to emit an ultrasonic wave which is coupled to the structure being inspected. The emitted wave enters the structure, where it is reflected by the crack. The return path of the reflected wave impinges on the transducer, where it is detected as a "pulse echo" signal.
The determination of the crack size, or depth of penetration in the case of surface-connected flaws, is a different and more complicated task. A conventional method for determining the depth of penetration of a planar crack is the back-scattered time-of-flight technique. This method takes advantage of the backward scattering of waves of ultrasonic energy at the edges of a crack. An emitter of short pulses of ultrasound, coupled to the inspection surface, causes refracted sound waves to impinge on the crack edge, which scatters the ultrasonic energy in all directions. A detector situated on the same or opposite surface as the crack is excited by scattered pulsed energy after a time delay. The time delay is a function of the crack height, the angle of refraction and other dimensions. By measuring the time-of-flight for the round trip from the transducer to the crack edge and back to the transducer, the crack height can be easily computed from the geometry.
Such ultrasonic inspections of the structural integrity of industrial components made of steel and other metals depend upon knowing the beam profiles of the ultrasonic waves that propagate into these components. It is common practice to control the refracted angle of the ultrasound by using hard shoes that follow the surface and maintain, more or less, a constant angle. Surfaces that are rough, with both short-term and long-term roughness, pose a problem because it is difficult to maintain contact. In addition, anisotropic materials, such as stainless steel weld metal and cast stainless steel components, redirect the ultrasound inside the material in an unpredictable manner.
Rough surface conditions and anisotropic grain structure can result in unpredictable results using conventional examination methods. Using the prior art, refracted angles are measured on a special calibration block, and then it is assumed that the angle is the same when applied to a specimen. Immersion inspection methods, which include tanks, booted search units, bubblers, and squirters, do not typically have a mechanism for maintaining a constant refracted angle.
The angle of refraction within a given material is controlled by the ultrasonic transducer's angle of incidence, i.e., the number of degrees by which the path of propagation is tilted relative to an axis normal to the object surface. The angle of incidence is determined in accordance with Snell's Law, which can be expressed mathematically as: EQU sin a/sin b=V.sub.1 /V.sub.2
where a is the angle of incidence; b is the angle of refraction; V.sub.1 and V.sub.2 are the respective wave velocities in the first and second media. Snell's Law describes wave behavior at an interface between two different media. The law applies even if mode conversion takes place.
FIG. 1 depicts a transducer 2 as it interrogates butt-welded stainless steel piping 8 and 8'. The refracted angle .theta. of the beam 4 impinging upon a defect 6 in the wall of pipe 8 is uncertain due to scattering at the rough surface 10 of weld overlay 14 and the interaction with the anisotropic columnar grain structure of the weld metal of butt weld 12 and weld overlay 14. The dashed line represents the weld fusion line. The angle of incidence is denoted by .alpha.. Numeral 16 denotes a counterbore or other geometrical reflector. Variation in the entry surface and the dendritic structure of the weld metal causes variations in the angle of the ultrasonic wavefront that impinges upon the crack, or other target, within the pipe wall. A couplant 18, such as water or oil, couples the ultrasound from transducer 2 to entry surface 10. The transducer may be a conventional flat-focused probe or a phased-array unit. In the event that a phased-array electromagnetic acoustical transducer is used, a couplant is not required.
Even perfect knowledge of the surface contour does not guarantee good results when inspecting with ultrasonic beams angled from the perpendicular. Wave interactions with the internal structure of metallic components can change the direction of propagation. For example, stainless steels with coarse columnar grain structures, commonly found in casting and weld metals, can redirect ultrasonic waves and result in misleading data. In some cases, the ultrasound is redirected to the opposite surface, resulting in a strong specular reflection and an apparent target that erroneously appears to be a mid-wall defect. Another common problem is confusion between reflections arising from longitudinal waves and those arising due to shear waves.
Booted search units or immersion methods are used if the surface is too irregular for contact methods. Immersion methods are used if components can be removed and placed in a water bath. Regardless of the means of dealing with surface conditions and anisotropic materials, a measurement of the exact refracted angle of the ultrasound is a necessary parameter for accurate sizing during pulse-echo examinations, and to verify coverage of the part.